In Swedish published application 348,831, filed Jan. 5, 1971, now abandoned, and in corresponding U.S. Pat. application Ser. No. 268,275, filed July 3, 1972, now abandoned, a method is described for contactless measurement of in particular movable wire in a wire-mill. This known measurement takes place so that light from a lamp is allowed to fall onto the wire in such a manner that the contours of the wire are projected onto an opto-electric component consisting of a number of photodiodes, the illumination of which is scanned successively in synchronism with a clock frequency, whereupon the signals from illuminated and non-illuminated photodiodes, respectively, are separated for the recording of the image position of the contours of the object.
Between the wire and the opto-electrical component in this known apparatus a lens system is arranged. This system of lenses brings about a projection of the contours of the wire onto the opto-electrical component.
In a further development of said known method the lens system comprises a telecentric diaphragm, whereby errors caused by the movements of the beam perpendicular to the optic axis of the lens system and perpendicular to the longitudinal direction of the wire are suppressed as shown in U.S. Pat. No. 3,874,798.